Often, a steering system of a vehicle which decreases a driver's operational steering wheel force to enable light and rapid steering operation uses hydraulic pressure to generate forces for assisting the steering power of the driver, also known as steering assistance power.
A hydraulic power steering system may include a power steering pump driven by the rotational power of an engine.
At stopping or at a low speed at which the friction force of a vehicle's tires is large, a rate of flow discharged from the pump must increase to generate large steering assistance power. Similarly, at a medium/high speed at which the friction force of the tire is small and safety is important, the rate of flow discharged from the pump must decrease to generate relatively low steering assistance power.
However, the power steering pump used in the hydraulic power steering system discharges a greater rate of flow at medium-high speeds than at low speeds due to the power steering pump rotating in direct proportion to an engine RPM. In such a situation, power (fuel efficiency) is lost.
Further, since the hydraulic power steering system cannot be electronically controlled, there are limitations on steering performance.
For example, when a steering operation is made to be lighter than at a normal time by compensating the steering power at a low-speed section, a convenient steering performance is possible, but the hydraulic power steering system in the related art cannot be controlled according to the vehicle speed.
To overcome the disadvantage, a vehicle speed sensitive motor driven power steering (MDPS), a system that assists steering only by torque of a motor without using the hydraulic pressure, has been developed and adopted. Such a system may change the steering power according to a driving vehicle speed.
The MOPS system is primarily mounted on a sedan in order to achieve a fuel efficiency improvement of the vehicle, since the hydraulic power steering system in the related art drives the power steering pump (hydraulic pump) by using the rotational power of the engine, and thus the engine power is unnecessarily consumed at all times. However, since the MDPS system consumes only the required energy at the moment when a steering performance is required by controlling the motor, the MDPS can contribute to a fuel efficiency improvement of the vehicle.
However, since a commercial vehicle such as a large-sized sedan or a truck has a larger weight than a general sedan, it is difficult to acquire required steering power only by the motor. As a result, an electrohydraulic power steering (EHPS) system that assists the steering power by using the hydraulic pressure generated by driving the hydraulic pump with the motor has been widely used.
FIG. 1 is a perspective view illustrating a general EHPS system and illustrates a steering wheel 1 for the steering operation, a steering angle sensor (SAS) 2 detecting a steering angle depending on a steering wheel operation (driver's input), a steering column universal joint 4, a motor pump unit (MPU) 5 generating and supplying the hydraulic pressure for assisting the steering power, a reservoir 6 storing steering oil and a gear box 7 performing vehicle steering using a driver's steering power transferred through the steering column 3 as well as the steering assistance power by the hydraulic pressure.
FIG. 2 is a configuration diagram schematically illustrating a configuration of the EHPS system in the related art. When the driver operates the steering wheel 1, a rack and pinion mechanism unit 8 converts rotational power transferred from the steering column 3 into a linear motion.
The rack and pinion mechanism unit 8 includes a rack gear 10 formed on a rack bar and a pinion gear 11 transferring power in abutment with the rack gear 10. When the driver rotates the steering wheel 1, rotational power is transferred to the pinion gear 11 through the steering column 3 and the transferred rotational power is converted into linear power by the pinion gear 11 and the rack gear 10 which moves the rack bar 9 linearly.
A motor pump unit 5 is driven and controlled according to a control signal output by an electric control unit (ECU), or, a steering controller 20, to generate and supply hydraulic pressure. The hydraulic pressure supplied by the motor pump unit 5 is supplied to a hydraulic cylinder 12 through a steering valve 15 controlling oil supply.
The hydraulic cylinder 12 assists the linear motion of the rack bar 9 by actuating the hydraulic pressure to generate the steering assistance power, and the steering valve 15 controls the flow of oil supplied to the hydraulic cylinder 12.
Reference numeral 14a represents an oil supply line to which oil is supplied by the motor pump unit 5, while reference numeral 14b represents an oil return line for returning oil to an oil reservoir 6. Reference numeral 13 represents a pressure relief valve (PRV).
In the EHPS system in the related art, the steering controller 20 controls a pump rotational speed (rpm) of the motor pump unit 5 based on a signal (vehicle speed signal) of a vehicle speed sensor and a signal (steering angle signal) of the steering angle sensor 2 to generate steering assistance power.
In this case, information regarding a steering angle (deg) depending on a steering wheel operation of a driver and a steering angular speed (deg/s) acquired by a differential value of the steering angle may be acquired from the signal of the steering angle sensor 2. The steering controller 20 controls the motor of the motor pump unit 5 based on the vehicle speed and the steering angular speed.
FIG. 3 is a flowchart illustrating a generation process of steering power (steering assistance power) of the EHPS system and FIG. 4 is a diagram illustrating one example of a map for a motor control of the motor pump unit.
First, when the vehicle speed detected by the vehicle speed sensor is input in the steering controller 20 while the vehicle is driven and a steering input is made by operating the steering wheel 1 by the driver (S1), the steering controller 20 acquires the steering angular speed based on the driver's steering input (steering wheel operation) from the signal of the steering angle sensor 2 (S2).
In this case, a motor rotational speed may be acquired from a map having the vehicle speed and the steering angular speed as inputs. The map may be a 3D map (motor RPM 3D map) in which the rotational speed is set as a value depending on the vehicle speed and the steering angular speed. In a vehicle development step, the map is prepared through an advance research and tuning process and thereafter, pre-stored in the steering controller of a produced vehicle.
Further, the steering controller 20 acquires the motor rotational speed of the motor pump unit 5 from the vehicle speed and the steering angular speed, driving motor M is controlled according to the acquired rotational speed (S3), and the motor pump unit 5 generates and supplies the hydraulic pressure (S4).
Consequently, while the hydraulic cylinder of the gear box is driven by the hydraulic pressure supplied from the motor pump unit (S5), the power generated assists the driver steering power depending on the steering wheel operation (S6).
Meanwhile, in the EHPS system in the related art, since a logic (passive boost logic) for controlling the motor rotational speed is applied only according to the vehicle speed and the steering angular speed as described above, even though a vehicle load increases or a slip moment increases due to low tire air pressure, the motor rotational speed is constant under conditions of the vehicle speed and the steering angular speed.
However, in this case, since a vehicle speed influences the motor rotational speed, the steering power of the driver increases.
As described above, in the EHPS system in the related art, since driving of the motor pump unit is controlled by a predetermined motor rotational speed only according to the vehicle speed and the steering angular speed, the steering power of the driver actually required may vary according to driving conditions such as a vehicle state, a road surface condition, aerodynamics acting on the vehicle, and the like. Consequently, a constant steering sense cannot be maintained in the steering wheel operation and the driver can feel a sense of difference or a catching sense.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.